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Olfactory expansion The Jurassic version of little red riding hood, is likely to have read 'Grandma, what a big brain you have,' to which the response would have been 'all the better to smell you with my dear.'

According to palaeontologists in the US, the brains of our ancient relatives initially expanded to facilitate a better sense of smell.

The findings, published today in the journal Science, put to rest some of the speculation on the evolutionary steps that led to the development of our large and complex mammalian brains.

The team, lead by Professor Tim Rowe, Director of the Vertebrate Paleontology Laboratory at the University of Texas at Austin, used x-ray computer tomography (CT) scans to digitally create three-dimensional images, called endocasts, of the brains of two of the earliest known 'pre-mammals', Morganucodon and Hadrocodium, from intact fossil skulls.

The endocasts show that the brain of Morganucodon is 50 per cent larger than that of the early primitive reptiles known as cynodonts, and that paper clip-sized Habercodium has a brain-to-body size similar to mammals.

In addition to approximating the size and shape of the brain, the endocasts provide anatomical information based on imprints left on the inner surfaces of the skull.

"Analysis of these surfaces reveal a lot of detail on the inner workings and circuitry of the brain," says Rowe. In this case, the researchers found the brain regions of our 190 million-year-old ancestors that had undergone the greatest expansion were those responsible for detecting and processing smell signals (the olfactory bulb and olfactory cortex).

"In Morganucodon and Hadrocodium, the olfactory bulb fills a chamber at the front of the brain. This changes the geometry of the brain; it looks a bit like a possum brain," says Rowe.

Based on comparisons with endocasts from other fossils and early mammals, the team found that the tiny shrew-like Morganucodon and Hadrocodium were also likely to have had improved coordination skills and a greater ability to interpret their environment via touch.

"It appears that there were two evolutionary pulses where the brain actually got bigger: the first where olfaction, tactile sensitivity [touch] and coordination dramatically improved [as seen with Morganucodon] and the second [exemplified by Hadrocodium], where this trend was taken even further," explains Rowe.

Rowe says there was also a third 'pulse' that didn't involve a change in brain size, rather an amplification of function, such as changes to the bones in the nose that support the olfactory membrane and a greater numbers of neurons in these regions.

Trade offs

The echidna, the closest living relative to Morganucodon and Hadrocodium, continued the evolutionary trend to enhance olfactory capabilities by developing electrosensors on the end of its nose.

But Rowe notes that many living mammals have improved on other senses at the expense of their ability to smell.

"There are many sensory modalities and different kinds of information out there: sound, smell, vision, geospatial, magnetic, electrical, but no animal has learned to optimise them all because there are limitations," he remarks.

"Humans gave up olfaction to gain really great eyes and ears. It's that trade off that is so interesting in the history of any particular lineage."